Managing foliar soybean diseases and fungicide applications

Often we are asked for the appropriate fungicide application timing for managing soybean diseases. Interestingly enough, that is a question that will be followed by additional questions from the plant pathologist. The reason for asking for more information is simple, but the answer might not be so cut and dry.

Those questions might be:

Which disease(s) are you concerned about?

What is the variety? (Is it susceptible to the pathogen of concern?)

Have you grown this variety before?

Was this disease present the last time you grew that variety?

What is the current growth stage of the crop?

Are you seeing symptoms of the disease at present or are you concerned about preventing an anticipated problem?

Is the field irrigated?

Have you used fungicides on this crop this season?

There could be others depending on the individual situation. Why are these questions asked? Because all of this information is important in answering the original question. A direct answer to the original question without additional information would be inappropriate.

Let us begin with the facts. Plant diseases may cause yield and quality losses in the soybean crop, but not all diseases are equal in importance. Disease management systems vary from one grower/region to another, but integrated approaches to management should be the norm. Integrated approaches include, disease resistance, cultural practices that might reduce disease development, and the use of fungicides.

No soybean variety has resistance to all diseases, but variety choice should include any available resistance to the main diseases of the geographic area where the crop is grown if the variety also possesses agronomic and yield characteristics desirable to the grower. Often fungicides are needed to protect yield and quality of the crop.

Current Considerations and Basic Principles

If a fungicide application is warranted, there are many considerations before choosing a product. Many fungicides are labeled for use in soybean. Basically, producers have five chemistry types from which to choose based on modeofaction. The Group 11 fungicides, or strobilurins (“strobies”), include but are not limited to products such as Aproach, Quadris, Headline, and Gem. The Group 7 fungicides, or SDHIs, include Endura, Vertisan, Priaxor, and others.

Group 3 fungicides, or triazoles, include products such as: Domark, Proline, and Topguard. A single Group 1 fungicide, thiophanatemethyl, is available to producers in several products (Topsin, Incognito, and Cercobin). A Group M5 fungicide, chlorothalonil, is available to producers in three products: Bravo, Echo, and Equus. All of the above-mentioned chemistry types are available in a number of premixes containing two different modesofaction.

Fungicide groups having a preventative effect on disease include the strobilurins (Group 11) and SDHIs (Group 7). Ideally, these products should be applied prior to disease development. Triazoles (Group 3) and chlorothalonil (Group M5) products have curative effects on diseases, halting or slowing existing disease development. Thiophanatemethyl (Group 1) is described as having both preventative and curative effects.

The strobilurins (Group 11), SDHIs (Group 7), and thiophanatemethyl (Group 1) have sitespecific modesofaction and should be used conservatively. These products can select for resistant pathogen populations very quickly. In fact, widespread resistance to strobilurin fungicides and thiophanatemethyl has been documented in the Cercospora leaf blight pathogen throughout soybeanproducing areas in Louisiana.

Concurrently, a decline in efficacy of strobilurins and thiophanatemethyl on Cercospora leaf blight has been observed over the past 1015 years. Additionally, strobilurin resistance has been confirmed in the frogeye leaf spot pathogen in several parishes throughout the state. Additional research is needed to determine the extent of this resistance.

Triazoles have a less specific modeofaction, but resistance is still possible and will occur much slower when compared to strobilurins. There have been no documented cases of resistance to chlorothalonil in soybean pathogens. Appropriate application timing also is an important consideration with fungicide applications to soybean. An automatic fungicide application at or near R1 is not currently recommended.

Ongoing research at LSU AgCenter may or may not result in modification of these recommendations in the future. More information is needed to determine if an R1 application timing is effective for producers statewide. According to results from many years of LSU research trials, fungicides have proven most efficacious when applied near the R3 stage. A second application may be considered at R5 as well.

Residual activity of fungicides and the timing between soybean growth stages are also an important consideration. Residual efficacy of fungicides varies, but the average residual of fungicides is 2 to 3 weeks.

However, residual activity does not simply cease after 2 to 3 weeks as if a light switch was thrown to the OFF position. Rather, we would expect a decline in activity until the concentration of the fungicide is no longer sufficient to suppress growth of the pathogen within the leaf. The analogy would be a light bulb getting dimmer with time until there is insufficient light to read. We would also expect that initial concentrations of the fungicide in the leaf and the rate of decline in activity (residual activity) would be a function of rate of application.

Keeping in mind that it takes about 10 weeks to progress from R1 to R6, and assuming a fungicide residual of 3 weeks, an application at R3 (pod development) would last until R5 (beginning pod fill). If a second fungicide was applied at R5, the material would last for most of the period until R6 (end pod fill). In this situation, plants would be protect-ed for approximately 6 of the 7.5 weeks of seed formation.

In addition to application timing, coverage and spray volume also are key components. Proper nozzle selection to obtain smaller droplet sizes will enhance coverage and, in turn, efficacy. Most fungicide labels recommend a minimum of 10 gallons/A spray volume by ground and 5 gallons/A by air. Finally, avoid unnecessary or excessive applications of fungicides, and always follow label directions for the best results.